The present invention relates to the field of radio communications, and, more particularly, to the calibration of radios.
In general, a radio system, e.g., an outdoor radio system, requires an accurate indication of both Receive Signal Level (xe2x80x9cRSLxe2x80x9d) and Output Power Indication (xe2x80x9cOPIxe2x80x9d) for field installation, service, and monitoring applications. U.S. Pat. Nos. 5,423,070 and 5,230,091 to Vaisanen et al., for example, are directed to tuning and compensating power levels in a radio telephone. The radio telephone includes sensors, connected to a processor, for detecting the radio""s operating channel and temperature ranges. The tuning of the radio is performed as a final stage in production. Specifically, external tuning equipment is connected to the radio to measure the power level of the transmitter. This power level is adjusted until the desired power level is achieved. Individual tuning results are stored in a memory as compensating values for use in controlling the transmitter power during operation. Thus, during operation, a logic controlled amplifier in the signal path amplifies the signal according to data corrected with the compensating values that correspond to the operating conditions.
U.S. Pat. No. 5,471,654 to Okazaki et al. discloses a radio capable of setting the level of power for transmitting an output signal, which correctly corresponds to the level of a received signal, without dependency upon change in the environmental temperature. The unit includes a temperature-dependent type automatic gain control (AGC) voltage generating circuit for converting a DC voltage (generated in proportion to a level of an output signal from a variable-gain amplifier) into an automatic gain control which depends on a change in environmental temperature. In other words, the level of the output AGC voltage is raised or lowered if the environmental temperature has been raised above or lowered below room temperature.
A temperature-compensated AGC circuit is also disclosed in U.S. Pat. No. 5,408,697 to Price. The circuit includes a gain independent compensation circuit for creating a second compensation signal responsive to temperature according to a predetermined characteristic. Specifically, the compensation circuit includes a thermistor to provide a receiver gain compensation signal to adjust the receiver amplifier gain responsive to temperature.
Also, U.S. Pat. No. 5,873,029 to Grondahl is directed to a millimeter wave power detector with temperature compensation. An RF power detector includes an RF detector circuit which converts an RF signal to a voltage representative of the RF signal""s power level. A temperature compensation element provides a temperature compensation signal to compensate for the temperature effects of the detector elements of the RF detector circuit.
Conventional calibration techniques include characterizing each radio over temperatures. The disadvantage of such an approach is that this requires extensive calibration time and usually becomes a bottleneck for mass production. For example, it may take 12 hours to calibrate a group of radios, with each calibrated over a temperature range.
In view of the foregoing background, it is therefore an object of the invention to provide a radio calibration method which reduces the time required for calibration.
This and other objects, features and advantages in accordance with the present invention are provided by a method for calibrating each radio of a plurality of radios at room temperature. More particularly, each radio preferably comprises a plurality of variable gain stages connected in series, at least one power detector connected to the variable gain stages, a temperature sensor, and a processor for controlling the variable gain stages in-service based upon the power detector, temperature sensor, and stored frequency and power compensation values. The method preferably includes generating frequency compensation values for at least one first variable gain stage by supplying a first calibration signal swept in frequency and while maintaining the radio at a constant temperature. The method may further include generating power compensation values for at least one second variable gain stage by supplying a second calibration signal swept in power level, and while maintaining the radio receiver at a constant temperature. The constant temperature is preferably room temperature, e.g. 65-75xc2x0 F. The frequency and power compensation values generated at the constant temperature are then stored for use by the processor during in-service temperature variations.
The method preferably includes the step of varying a temperature for at least one radio while performing the generating steps to generate temperature factors to be used for radios calibrated at the constant temperature. The temperature factors may be used to generate the frequency and power compensation values. Also, the temperature factors may be stored for use by the processor in combination with the frequency and power compensation values during in-service temperature variations.
The method may further include the step of maintaining the at least one second variable gain stage in a predetermined linear portion of a dynamic range thereof during the step of generating frequency compensation values. Also, the at least one first variable gain stage may be downstream from the at least one second variable gain stage and each variable gain stage may comprise at least one variable attenuator.
The radio may further include a receiver signal level indicator for indicating the received signal level in-service based upon the frequency and power compensation values. Also, a constant power level is maintained while generating the frequency and power level compensation values. The calibration method may be performed on a receiver, transmitter or both, of the radio. Furthermore, for the receiver, the method may include the step of maintaining the at least one second variable gain stage in a predetermined linear portion of a dynamic range thereof during the step of generating frequency compensation values.
For the transmitter, the method may further include the step of setting the second at least one variable gain stage to produce a nominal power level output. Also, the transmitter may further comprise at least one mixer upstream of the first and second gain stages and at least one gain stage upstream of the at least one mixer. Accordingly, the method may further include the step of verifying operation of the at least one third gain stage in combination with the processor.